Xenotransplantation: the challenge to current psychosocial attitudes

Because of the limited availability of transplantable human organs, xenotransplantation, the use of animal organs as an alternative source, has received considerable attention in recent years. Xenotransplantation would provide an unlimited supply of organs, and these organs would be available whenever required. Although the pig is considered the best source for organs, significant immunologic barriers currently prohibit the implementation of a clinical trial of organ transplantation. However, as medical research gains more insight into the mechanisms underlying rejection of pig organs in primates, therapeutic xenotransplantation is becoming more feasible. Clinical trials of porcine cell transplants are currently underway. Although xenotransplantation will minimize the waiting period for an organ and obviate the feelings of guilt or indebtedness commonly experienced by recipients of human organs, several psychosocial issues may hinder the reintegration of patients into society. For example, concerns that infectious pathogens could be transferred to recipients of pig organs will necessitate life-long monitoring and perhaps even temporary isolation of patients. The possible risk of the spread of a xenozoonosis from the patient to other members of the community may inspire public controversy and even fear, which may have an adverse impact on the patient's emotional state. Additionally, some patients may be psychologically disturbed by the need to incorporate pig organs into their body. This article addresses these and other psychosocial issues that may be associated with clinical xenotransplantation.

Report of the Xenotransplantation Advisory Committee of the International Society for Heart and Lung Transplantation: the present status of xenotransplantation and its potential role in the treatment of end-stage cardiac and pulmonary diseases

An urgent and steadily increasing need exists world-wide for a greater supply of donor thoracic organs. Xenotransplantation offers the possibility of an unlimited supply of hearts and lungs that could be available electively when required. However, anti-body- mediated mechanisms cause the rejection of pig organs transplanted into non-human primates, and these mechanisms provide major immunologic barriers that have not yet been overcome. Having reviewed the literature on xenotransplantation, we present a number of conclusions on its present status with regard to thoracic organs, and we make a number of recommendations relating to eventual clinical trials. Although pig hearts have functioned in heterotopic sites in non-human primates for periods of several weeks, median survival of orthotopically transplanted hearts is currently ,1 month. No transplanted pig lung has functioned for even 24 hours. Current experimental results indicate that a clinical trial would be premature. A potential risk exists, hitherto undetermined, of transferring infectious organisms along with the donor pig organ to the recipient, and possibly to other members of the community. A clinical trial of xeno-transplantation should not be undertaken until experts in microbiology and the relevant regulatory authorities consider this risk to be minimal. A clinical trial should be considered when approximately 60% survival of life-supporting pig organs in non-human primates has been achieved for a minimum of 3 months, with at least 10 animals surviving for this minimum period. Furthermore, evidence should suggest that longer survival (.6 months) can be achieved. These results should be achieved in the absence of life-threatening complications caused by the immunosuppressive regimen used. The relationship between the presence of anti-HLA antibody and anti-pig antibody and their cross-reactivity, and the outcome of pig-organ xenotransplantation in recipients previously sensitized to HLA antigens require further investigation. We recommend that the patients who initially enter into a clinical trial of cardiac xenotransplantation be unacceptable for allotransplantation, or acceptable for allotransplantation but unlikely to survive until a human cadaveric organ becomes available, and in whom mechanical assist-device bridging is not possible. National bodies that have wide-reaching government-backed control over all aspects of the trials should regulate the initial clinical trial and all subsequent clinical xenotransplantation procedures for the foreseeable future. We recommend coordination and monitoring of these trials through an international body, such as the International Society for Heart and Lung Transplantation, and setting up a registry to record and widely disperse the results of these trials. Xenotransplantation has the potential to solve the problem of donor-organ supply, and therefore research in this field should be actively encouraged and supported.

Coagulation and thrombotic disorders associated with pig organ and hematopoietic cell transplantation in nonhuman primates

BACKGROUND

Efforts to achieve tolerance to transplanted pig organs in nonhuman primates by the induction of a state of mixed hematopoietic chimerism have been associated with disorders of coagulation and thrombosis. Activation of recipient vascular endothelium and platelets by porcine hematopoietic cells and/or activation of donor organ vascular endothelium and/or molecular differences between the species may play roles. Irradiation or drug therapy could possibly potentiate endothelial cell activation and/or injury.

METHODS

We have investigated parameters of coagulation and platelet activation in nonhuman primates after (1) a regimen aimed at inducing mixed hematopoietic chimerism and tolerance (TIR that included total body irradiation, T cell depletion, and splenectomy; (2) pig bone marrow or pig peripheral blood mobilized progenitor cell transplantation (PCTx); and/or (3) pig organ transplantation (POTx). Five experimental groups were studied. Baboons were the recipient subjects in all groups except Group 1. Gp 1 Cynomolgus monkeys (n=6) underwent TIR + allotransplantation of hematopoietic cells and a kidney or heart or TIR + concordant xenotransplantation (using baboons as donors) of cells and a kidney; Gp 2 Baboons (n=4) underwent TIR with or without (+/-) autologous hematopoietic cell infusion; Gp 3 (n=12) PCTx+/-TIR; Gp 4 (n=5) POTx+/-TIR; Gp 5 (n=4) TIR + PCTx + POTx. Platelet counts, with plasma prothrombin time, partial thromboplastin time, fibrinogen levels, fibrin split products and/or D-dimer were measured.

RESULTS

In the absence of a discordant (porcine) cellular or organ transplant (Groups 1 and 2), TIR resulted in transient thrombocytopenia only, in keeping with bone marrow depression from irradiation. PCTx alone (Group 3) was associated with the rapid development of a thrombotic thrombocytopenic (TTP)-like microangiopathic state, that persisted longer when PCTx was combined with TIR. POTx (+/-TIR) (Group 4) was associated with a gradual fall (over several days) in platelet counts and fibrinogen with disseminated intravascular coagulation (DIC); after graft excision, the DIC generally resolved. When TIR, PCTx and POTx were combined (Group 5), an initial TTP-like state was superseded by a consumptive picture of DIC within the first week, necessitating graft removal.

CONCLUSIONS

Both PCTx and POTx lead to profound alterations in hemostasis and coagulation parameters that must be overcome if discordant xenotransplantation of hematopoietic cells and organs is to be fully successful. Disordered thromboregulation could exacerbate vascular damage and potentiate activation of coagulation pathways after exposure to xenogeneic cells or a vascularized xenograft.

Inhibition of platelet aggregation in baboons: therapeutic implications for xenotransplantation

Activation of endothelial cells and platelet sequestration play major roles in rejection of xenografts. The histopathology of both hyperacute and acute vascular or delayed rejection of vascularized discordant xenografts is characterized by interstitial hemorrhage and intravascular thrombosis. Agents that prevent platelet activation and consequent microthrombus formation have proven beneficial in xenograft rejection but do not fully preclude vascular thrombosis. Recently, several new anti-platelet therapies have undergone extensive clinical testing for atherosclerotic thrombotic vascular disorders; other putative therapies are undergoing pre-clinical evaluation. We have investigated the effect of several of these novel agents on platelet aggregation in baboons in order to screen for future potential in xenograft rejection models.

METHODS

Drugs tested in these experiments were aurintricarboxylic acid (ATA, von Willebrand Factor-GPIb inhibitor), fucoidin (a selectin-inhibitor), 1-benzylimidazole (1-BI, thromboxane synthase antagonist), prostacyclin (PGI2, endothelial stabilizer), heparin (thrombin antagonist), nitroprusside sodium or nicotinamide (NPN or NA, both NO-donors), and eptifibatide (EFT, GPIIb/IIIa receptor antagonist). These were infused intravenously to nine baboons. Coagulation parameters and platelet counts were monitored and baboons were observed for adverse side-effects. The efficacy of these agents in inhibiting platelet aggregation was assayed in a platelet aggregometer.

RESULTS

Treatment with ATA and fucoidin resulted in complete inhibition of platelet aggregation but also in major perturbation of coagulation parameters. 1-BI and PGI2 had no effect when administered alone, but in combination resulted in moderate inhibition of aggregation without disturbance in PT or PTT. NPN and NA had no substantive effects on platelet aggregation. Heparin resulted in specific inhibition of thrombin-induced platelet aggregation and, as anticipated, was associated with moderate prolongation of PTT. Importantly, EFT caused complete inhibition of platelet aggregation without changes in coagulation. Platelet counts, fibrinogen levels, and fibrinogen degradation products remained within the normal ranges in all experiments.

CONCLUSIONS

Although excellent inhibition of platelet activation was obtained with ATA and fucoidin, clinical use may be precluded by concomitant disturbances of coagulation. Combinations of heparin and EFT may prove beneficial in preventing the thrombotic disorders associated with xenograft rejection while maintaining adequate hemostatic responses. These agents are to be evaluated in our pig-to-primate xenotransplantation models.

Plasma perfusion by apheresis through a Gal immunoaffinity column successfully depletes anti-Gal antibody: experience with 320 aphereses in baboons

BACKGROUND

Anti-Galalpha1-3Gal (Gal) antibodies (Gal Ab) contribute to the rejection of porcine organs transplanted into primates. Extracorporeal immunoadsorption (EIA) has been developed to eliminate Gal Ab from the circulation.

METHODS

Between 1995 and 1999 we performed 320 EIAs in baboons using a COBE-Spectra apheresis unit incorporating a synthetic Gal immunoaffinity column. Three plasma volumes were immunoadsorbed on each occasion. The 221 consecutive EIAs performed in 41 immunosuppressed baboons between January 1997 and April 1999 form the basis of this review. Of these 41 baboons, 29 underwent a series of three or four EIAs at daily intervals, seven had multiple series of three EIAs, and the remainder underwent single or double EIAs. Serum Gal Ab levels were monitored by ELISA before and at intervals after the course of EIA.

RESULTS

There were two fatal complications, one from a respiratory mishap (unrelated to the EIA) and one from persistent hypotension unresponsive to therapeutic interventions. Seven procedures (3%) were terminated early owing to technical difficulties and/or persistent hypotension. Mean pre-EIA Gal Ab levels in naive baboons were 33.1 microg/ml (IgM) and 14.5 microg/ml (IgG). Immediately after three consecutive EIAs, IgM was depleted by a mean of 97.3% and IgG by 99.4%. By 18 to 24 h later, Gal Ab was returning but depletion remained at 80.1% (IgM) and 84.7% (IgG). The subsequent rate of return of Gal Ab depended on the immunomodulatory protocol used.

CONCLUSIONS

(1) With appropriate monitoring, EIA is an acceptably safe procedure, even in small (<10 kg) baboons. (2) Three consecutive EIAs are effective in removing >97% of Gal Ab. (3) In the majority of cases, return of Gal Ab begins within 24 h, irrespective of the immunomodulatory protocol.

Disordered regulation of coagulation and platelet activation in xenotransplantation

Rejection of xenografts is associated with vascular-based inflammation, thrombocytopenia and the consumption of coagulation factors that may evolve into disseminated intravascular coagulation (DIC). Similarly, bone marrow-derived cellular xenotransplantation procedures are associated with endothelial cell activation and thrombotic microangiopathic injury. These complications generally develop despite the best available measures for depletion of xenoreactive natural antibody, inhibition of complement activation and suppression of T- and B-cell mediated immune responses. The mechanisms underlying the DIC and thrombotic microangiopathy associated with xenotransplantation are unclear. A proposed primary biological dysfunction of xenografts with respect to regulation of clotting could amplify vascular injury, promote immunological responses and independently contribute to graft failure. Disordered thromboregulation could have deleterious effects, comparable to unregulated complement activation, in the pathogenesis of xenograft rejection and may therefore represent a substantive barrier to xenotransplantation.

Oesophageal syncope

Oesophageal syncope is loss of consciousness on swallowing, which is thought to be associated with an abnormal vagovagal reflex that leads to transient bradycardia. Patients with this potentially lethal condition may present to neurologists, cardiologists or gastroenterologists. It may be associated with cardio-active drug therapy, previous myocardial infarction, or with an organic lesion of the lower oesophagus. Barium and manometric studies, in association with ECG monitoring, should, therefore, be carried out in all cases. In many patients, however, it appears to be a functional abnormality for which no cause can be determined. In the absence of a condition necessitating surgical correction, medical therapy in the form of anticholinergic or sympathomimetic agents is occasionally helpful. Total denervation of the affected portion of the oesophagus has successfully prevented further symptoms, but insertion of a programmed cardiac pacemaker is currently the definitive treatment of choice.

High-dose porcine hematopoietic cell transplantation combined with CD40 ligand blockade in baboons prevents an induced anti-pig humoral response

BACKGROUND

In pig-to-primate organ transplantation, hyperacute rejection can be prevented, but the organ is rejected within days by acute vascular rejection, in which induced high-affinity anti-Gal alpha1-3Gal (alphaGal) IgG and possibly antibodies directed against new porcine (non-alphaGal) antigenic determinants are considered to play a major role. We have explored the role of an anti-CD40L monoclonal antibody in modifying the humoral response to porcine hematopoietic cells in baboons pretreated with a nonmyeloablative regimen.

METHODS

Porcine peripheral blood mobilized progenitor cells obtained by leukapheresis from both major histocompatibility complex-inbred miniature swine (n=7) and human decay-accelerating factor pigs (n=3) were transplanted into baboons. Group 1 baboons (n=3) underwent whole body (300 cGy) and thymic (700 cGy) irradiation, T cell depletion with ATG, complement depletion with cobra venom factor, short courses of cyclosporine, mycophenolate mofetil, porcine hematopoietic growth factors, and anti-alphaGal antibody depletion by immunoadsorption before transplantation of high doses (2-4 x 10(10)/cells/kg) of peripheral blood mobilized progenitor cells. In group 2 (n=5), cyclosporine was replaced by eight doses of anti-CD40L monoclonal antibodies over 14 days. The group 3 baboons (n=2) received the group 1 regimen plus 2 doses of anti-CD40L monoclonal antibodies (on days 0 and 2).

RESULTS

In group 1, sensitization to alphaGal (with increases in IgM and IgG of 3- to 6-fold and 100-fold, respectively) and the development of antibodies to new non-alphaGal porcine antigens occurred within 20 days. In group 2, no sensitization to alphaGal or non-alphaGal determinants was seen, but alphaGal-reactive antibodies did return to their pre- peripheral blood mobilized progenitor cells transplant levels. In group 3, attenuated sensitization to alphaGal antigens was seen after cessation of cyclosporine and mycophenolate mofetil therapy at 30 days (IgM 4-fold, IgG 8-30-fold), but no antibodies developed against new porcine determinants. In no baboon did anti-CD40L monoclonal antibodies prevent sensitization to its own murine antigens.

CONCLUSIONS

We believe these studies are the first to consistently demonstrate prevention of a secondary humoral response after cell or organ transplantation in a pig-to-primate model. The development of sensitization to the murine elements of the anti-CD40L monoclonal antibodies suggests that nonresponsiveness to cell membrane-bound antigen (e.g., alphaGal) is a specific phenomenon and not a general manifestation of immunological unresponsiveness. T cell costimulatory blockade may facilitate induction of mixed hematopoietic chimerism and, consequently, of tolerance to pig organs and tissues.

Acute humoral xenograft rejection: destruction of the microvascular capillary endothelium in pig-to-nonhuman primate renal grafts

The major cause of xenograft loss beyond hyperacute rejection is a form of injury, traditionally termed delayed xenograft rejection (DXR), whose pathogenesis is unknown. Here we analyze the immunologic and morphologic features of DXR that develops in pig kidney xenografts transplanted into nonhuman primates. Kidneys from miniature swine were transplanted into cynomolgus monkeys (n = 14) or baboons (n = 11) that received regimens aimed to induce mixed chimerism and tolerance. No kidney was rejected hyperacutely. Morphologic and immunohistochemical studies were performed on serial biopsies, and an effort was made to quantify the pathologic features seen. The early phase of DXR (Days 0-12) was characterized by focal deposition of IgM, IgG, C3, and scanty neutrophil and macrophage infiltrates. The first abnormality recognized was glomerular and peritubular capillary endothelial cell death as defined by in situ DNA nick-end labeling (TUNEL). Damaged endothelial cells underwent apoptosis and, later, frank necrosis. The progressive phase developed around Day 6 and was characterized by progressive deposition of IgM, IgG, C3, and prominent infiltration of cytotoxic T cells and macrophages, with a small number of NK cells. Thrombotic microangiopathy developed in the glomeruli and peritubular capillaries with TUNEL+ endothelial cells, platelet aggregation, and destruction of the capillary network. Only rare damaged arterial endothelial cells and tubular epithelial cells were observed, with rare endothelialitis and tubulitis. In the advanced phase of DXR, interstitial hemorrhage and infarction occurred. During the development of DXR, the number of TUNEL+ cells increased, and this correlated with progressive deposition of antibody. The degree of platelet aggregation correlated with the number of TUNEL+ damaged endothelial cells. We conclude that peritubular and glomerular capillary endothelia are the primary targets of renal DXR rather than tubular epithelial cells or arterial endothelium and that the earliest detectable change is endothelial cell death. DXR was characterized by progressive destruction of the microvasculature (glomeruli and peritubular capillaries) and formation of fibrin-platelet thrombi. Both cytotoxic cells and antibodies potentially mediate the endothelial damage in DXR; however, in this model, DXR is largely humorally mediated and is better termed "acute humoral xenograft rejection."

Gene therapy and transplantation

Advances in molecular biology and in techniques of gene transfer have resulted in the development of practical approaches to human gene therapy. Many applications are of relevance to manipulation of the immune system and have potential in organ and cell transplantation. For example, gene therapy approaches may facilitate the induction of immunological tolerance to a donor organ or protect it locally against the host's immune response. Based on a comprehensive review of the world literature, examples of current research efforts in both allogeneic and xenogeneic transplantation are presented and discussed.

An approach to the control of disease transmission in pig-to-human xenotransplantation

Although several major immunologic hurdles need to be overcome, the pig is currently considered the most likely source animal of cells, tissues and organs for transplantation into humans. Concerns have been raised with regard to the potential for the transfer of infectious agents with the transplanted organ to the human recipient. This risk is perceived to be increased as it is likely that the patient will be iatrogenically immunocompromised and the organ-source pig may be genetically engineered in such a way to render its organs particularly susceptible to infection with human viruses. Furthermore, the risk may not be restricted to the recipient, but may have consequences for the health of others in the community. The identification of porcine endogenous retroviruses and of hitherto unknown viruses have given rise to the most concern. We document here the agents we believe should be excluded from the organ-source pigs. We discuss the likelihood of achieving this aim and outline the potential means by which it may best be achieved.

Pharmacotherapeutic agents in xenotransplantation

The ability to transplant pig organs into humans would resolve the current crisis in the supply of cadaveric human organs for the treatment of end stage disease. Several immunologic barriers need to be overcome if pig-to-primate transplantation is to be successful. The presence of preformed antibodies in humans, apes and Old World monkeys directed against galactose epitopes on pig vascular endothelium provides the major barrier, as binding of antibody to antigen leads to graft destruction by complement activation and other mechanisms. Hyperacute rejection can result from the action of complement. If this is prevented, delayed antibody-mediated rejection develops, which can be associated with a state of consumptive coagulopathy (disseminated intravascular coagulation, DIC). Efforts being made to overcome antibody-mediated rejection include depletion of antibody by extracorporeal immunoadsorption, prevention of an induced antibody response by co-stimulatory blockade, B-cell and/or plasma cell depletion, depletion or inhibition of complement, or the use of organs from pigs transgenic for a human complement regulatory protein, such as hDAF. The ultimate solution would be the induction of both B- and T-cell tolerance to the transplanted pig organ, which is being explored by attempting to induce haematopoietic cell chimerism. One complication of this is a thrombotic microangiopathy, similar to thrombotic thrombocytopenic purpura. The many and diverse roles in which pharmacotherapy is involved in attempts to overcome the barriers of xenotransplantation are reviewed and current progress, particularly in our own laboratory, is discussed.